Misconception: It’s just a theory.

Actually: Theories are neither hunches nor guesses. They are the crown jewels of science.

One day, it’s Megyn Kelly who has a theory about why Donald J. Trump hates her.

Another day, the newly released trailer for the next Star Wars movie inspires a million theories from fans about who Rey’s parents are.

And on Twitter, someone going by the name of Mothra P.I. has a theory about how cats can assume a new state of matter:

In everyday conversation, we tend to use the word “theory” to mean a hunch, an idle speculation, or a crackpot notion.

That’s not what “theory” means to scientists.

“In science, the word theory isn’t applied lightly,” Kenneth R. Miller, a cell biologist at Brown University, said. “It doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

Dr. Miller is one of the few scientists to have explained the nature of theories on a witness stand under oath.

He is a co-author of a high school biology textbook that puts a strong emphasis on the theory of evolution. In 2002, the board of education in Cobb County, Ga., adopted the textbook but also required science teachers to put a warning sticker inside the cover of every copy.

“Evolution is a theory, not a fact, regarding the origin of living things,” the sticker read, in part.

In 2004, several Cobb County parents filed a lawsuit against the county board of education to have the stickers removed. They called Dr. Miller, who testified for about two hours, explaining, among other things, the strength of evidence for the theory of evolution.

Once the lawyers had finished questioning Dr. Miller, he stepped down from the stand and made his way out of the courtroom. On the way, he noticed a woman looking him straight in the eye.

“She said, ‘It’s only a theory, and we’re going to win this one,’” Dr. Miller recalled.

They didn’t. In 2005 the judge ruled against the board of education. The board appealed the decision but later agreed to remove the stickers.

Peter Godfrey-Smith, the author of “Theory and Reality: An Introduction to the Philosophy of Science,” has been thinking about how people can avoid the misunderstanding embedded in the phrase, “It’s only a theory.”

It’s helpful, he argues, to think about theories as being like maps.

“To say something is a map is not to say it’s a hunch,” said Dr. Godfrey-Smith, a professor at the City University of New York and the University of Sydney. “It’s an attempt to represent some territory.”

A theory, likewise, represents a territory of science. Instead of rivers, hills, and towns, the pieces of the territory are facts.

“To call something a map is not to say anything about how good it is,” Dr. Godfrey-Smith added. “There are fantastically good maps where there’s not a shred of doubt about their accuracy. And there are maps that are speculative.”

To judge a map’s quality, we can see how well it guides us through its territory. In a similar way, scientists test out new theories against evidence. Just as many maps have proven to be unreliable, many theories have been cast aside.

But other theories have become the foundation of modern science, such as the theory of evolution, the general theory of relativity, the theory of plate tectonics, the theory that the sun is at the center of the solar system, and the germ theory of disease.

“To the best of our ability, we’ve tested them, and they’ve held up,” said Dr. Miller. “And that’s why we’ve held on to these things.”

A version of this article appears in print on , on Page D6 of the New York edition with the headline: Educated Guesses: In Science, It’s Never ‘Just a Theory’. Order Reprints | Today’s Paper | Subscribe